The present invention relates to a superconducting transformer and, more particularly, to a superconducting transformer for transferring electric energy from a primary superconducting coil to a secondary superconducting coil by means of magnetic induction.
This superconducting transformer includes primary and secondary superconducting coils made of superconducting wires, a core a portion of which is coaxial with the coils, thereby forming a closed magnetic circuit, and a cryostat for maintaining the superconducting coils in a superconducting state. The superconducting coils and the core are immersed in a refrigerant, such as liquid helium, contained in the cryostat. More specifically, the superconducting coils are cooled by the refrigerant, thereby to be assume a superconducting state.
Superconducting wires in the superconducting state experience almost no hysteresis loss or coupling loss. That is, superconducting coils in the superconducting state experience almost no ohmic loss. Thus, a superconducting transformer can transfer a large quantity of electric energy from the primary coil to the secondary coil, with almost no AC loss.
However, even if it is not necessary to place the core in the superconducting state, the core is nevertheless immersed in the refrigerant. When the superconducting transformer is operated, Joule heat is generated from the core, due to the iron loss thereof. Thus, when the core is immersed in the refrigerant, a large quantity of refrigerant is heated by the Joule heat, with the result that the cooling efficiency of the superconducting coils is inevitably degraded. Consequently, it is necessary that the core be insulated from the refrigerant.
As is shown in FIG. 1, a superconducting transformer has been proposed which satisfies this requirement. A cryostat includes vessel 1, which contains heat insulating tank 2. Vessel 1 has annular side wall 3, bottom wall 4, and cover 5. Core 8 is annular in shape, is formed of a rod, and has first portion 11, second portion 12, third portion 13, and fourth portion 14. First portion 11 is inserted in opening 6 of bottom wall 4 and opening 7 of cover 5. Second to fourth portions 12 to 14 are located outside vessel 1. Heat insulating tank 2 has side 15 and bottom 16 extending along the inner surfaces of side wall 3 and bottom wall 4, respectively, and enclosure 17 extending along the outer periphery of first portion 11 of annular core 8. Primary and secondary superconducting coils 9 and 10 are provided which are coaxial with first portion 11 of annular core 8 and surround enclosure 17. Refrigerant is contained in heat insulating tank 8.
Therefore, superconducting coils 9 and 10 are magnetically coupled with first portion 11 of core 8. First portion 11 of core 8 is thermally insulated from the refrigerant by enclosure 17 of heat insulating tank 2, without coming into contact with the refrigerant. Second to fourth portions 12 to 14 of core 8 are located outside of vessel 1, and not in contact with the refrigerant. In this way, the requirement that the core be insulated from the refrigerant, is satisfied.
In the case of the superconducting transformer shown in FIG. 1, before core 8 is bent, it is passed through opening 6 of bottom wall 4, coils 9 and 10, enclosure 17, and opening 7 of cover 5. Thereafter, core 8 is bent annularly. Thus, after core 8 has been formed into an annular shape, it is then difficult to remove superconducting coils 9 and 10 therefrom, for maintenance. If it is necessary to remove superconducting coils 9 and 10 from annular core 8, core 8 must then be cut open. Consequently, maintenance and inspection of the superconducting coils cannot be performed with ease.
Since second to fourth portions 12 to 14 of core 8 are disposed outside the vessel, it is necessary to increase the mechanical strengths of the vessel and the heat insulating tank. Particularly, since the load of core 8 is applied to the upper and lower ends of enclosure 17 of heat insulating tank 2, the upper and lower ends of enclosure 17 must be formed rigidly. Thus, the thickness of enclosure 17 of heat insulating tank 2 is increased. Consequently, a space between superconducting coils 9, 10 and first portion 11 of core 8 is increased. Therefore, leakage magnetic flux radiated from superconducting coils 9, 10 through the space is increased. As a result, the transferring loss of the superconducting transformer is increased.